The present invention relates to high speed generators and, more particularly, to high speed generators used with gas turbine engines such as those used in aircraft, tanks, ships, terrestrial, or other applications.
Generator systems for aircraft may include three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter, and a main generator. The PMG includes permanent magnets on its rotor. When the PMG rotates, AC currents are induced in stator windings of the PMG. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current. This DC current next is provided to stator windings of the exciter. As the rotor of the exciter rotates, three phases of AC current are typically induced in the rotor windings. Rectifier circuits that rotate with the rotor of the exciter rectify this three-phase AC current, and the resulting DC currents are provided to the rotor windings of the main generator. Finally, as the rotor of the main generator rotates, three phases of AC current are typically induced in its stator windings, and this three-phase AC output can then be provided to a load such as, for example, an aircraft electrical system.
Because some aircraft generators are high speed generators with potential rotational speeds up to and in excess of 24,000 rpm, potentially large centrifugal forces may be imposed upon the rotors in generators. Given these potentially stressful operating conditions, the rotors should be carefully designed and manufactured, so that the rotors are reliable and precisely balanced. Improper balancing not only can result in inefficiencies in the operation of a generator, but may also affect the reliability of in the generator.
Among the components of a rotor that provide increased reliability and proper balancing of the rotors are the wire coils wound on the rotor. The centrifugal forces experienced by a rotor may be strong enough to cause bending of the wires of these coils into what is known as the interpole region. Over time, such bending can result in mechanical breakdown of the wires and compromise of the coil insulation system. Additionally, because the coils are assemblies of individual wires that can move to some extent with respect to one another and with respect to the remaining portions of the rotors, the coils are a potential source of imbalance within the rotor and can potentially compromise the insulation system. Even asymmetrical movements of these coils on the order of only a few thousandths of an inch can, in some instances, be significant.
In order to improve the strength and reliability of the wire coils and the coil insulation system, and to minimize the amount of imbalance in the rotors that may occur due to the wire coils, the rotors may include a coil retention system. With a coil retention system, substantially rigid wedges are inserted in between neighboring poles of the rotors to reduce the likelihood of coil wire bending or movement. In some embodiments, the wedges may also exert some force onto the coils to help maintain the physical arrangement of the coils.
Although the wedges employed in conventional coil retention systems are capable of providing the above-described benefits to some extent, the design of these conventional wedges also limits their effectiveness. In particular, the loading on the wedges and other components used in a conventional retention system may be affected by tolerance accumulation and assembly variations, which can result in coil movement and loading regimes that can adversely affect generator performance and reliability. Because conventional wedges may be made of metal and hence are rigid, they are less likely to alleviate any of these potential tolerance accumulations and assembly variations.
From the foregoing, there is a need for a system and method of providing coil retention in high speed generators that addresses the one or more of the above-noted drawbacks. Namely, there is a need for a coil retention system and method that will work under circumstances involving tolerance accumulation and/or assembly variations in the rotor. The present invention addresses this need.